1. Field of the Invention
This invention relates to an optical recording medium using an azo metal complex dye in a recording layer.
2. Prior Art
The present inventors have been engaged in the development of write-once type compact discs (CD-R) as a recordable optical recording medium corresponding to the compact disc (CD) standard. Higher density optical recording media are desired in the recent years, and some of such media heretofore proposed are CD-R of the next generation wherein the recording wavelength of CD-R is reduced from the current 780 nm to 680 nm to 635 nm and write-once type digital video discs (DVD-R). However, since development works have hitherto been made so as to match with 780 nm, there are known few dyes which can satisfy desired characteristics including light resistance, solubility and recording sensitivity on the shorter wavelength side of 680 nm to 635 nm. A further noteworthy requirement is interchangeability with the current standard. More particularly, unless the media which have been recorded by the existing recording device are at least readable with shorter wavelength light, there results a barrier against simple and rapid utilization of a pile of past accumulated information. For short wavelength recording layers to be developed from now on, it is considered important that they not only satisfy the standard at the current standard for wavelength 780 nm, known as the Orange Book standard, but also exhibit equivalent characteristics to the current standard even at 680 nm to 635 nm.
One measure contemplated to realize this demand in a most practical way is by mixing a dye for a short wavelength with a proper amount of a dye having satisfactory characteristics on the current standard, but free of substantial absorbance on the short wavelength side so as to satisfy characteristics at both the wavelengths. In this case, recording characteristics of a mixed system must be fully investigated.
For a dye to be used in recording layers, one characteristic which is especially important and difficult to accomplish is light resistance although the recording wavelength is important. Heretofore, dyes belonging to metal complex systems have been disclosed as highly light resistant dyes. As disclosed in JP-A 55795/1984, for example, there is known an example of combining a cyanine dye having low light resistance with a metal complex quencher for improving light resistance. This system, however, has the drawbacks that it has a drastically reduced solubility in a coating solvent used for spin coating and that the stabilizer itself can be decomposed and degraded. Other dyes having high light resistance include, for example, azo metal complex dyes (JP-B 51682/1995) and formazane nickel complex dyes (JP-A 254038/1985 and 144997/1987). Although these metal complex dyes surely have excellent light resistance, they generally have drawbacks of low recording sensitivity, relatively low solubility, and solubility in only limited solvents. When write/read operation at a short wavelength of 680 nm to 635 nm is taken into account, the formazane nickel complex dyes have a too long absorption wavelength, suggesting that this skeleton can no longer accommodate shorter wavelengths.
With respect to the phthalocyanine dyes known as highly light resistant dyes from the past, they are also difficult to avoid the above-mentioned drawbacks and thus deemed impossible to expect further development as a dye appropriated for short wavelength recording layers. As mentioned above, there is a strong demand for a dye skeleton which possesses desired write/read characteristics at a short wavelength of 680 nm to 635 nm, especially when used in a mixed system with a dye having absorbance on the long wavelength side, and which has high light resistance and solubility by itself.